1. Field of the Invention
The present invention relates to an optical filter applicable to a gain equalizer for equalizing the gain of an optical fiber amplifier as well, an optical amplifier system including the same, and an optical communication system including the optical amplifier system.
2. Related Background Art
In optical communication systems, an Er-doped optical fiber amplifier (EDFA: Erbium-Doped Fiber Amplifier) has been utilized as an optical amplifier for amplifying respective signal powers of a plurality of channels included in a communication wavelength band. The EDFA uses an optical fiber having a light-guiding region doped with Er (EDF: Erbium-Doped Fiber) as an amplifying medium, and supplies pumping light (at a wavelength of 1.48 xcexcm or 0.98 xcexcm) to the EDF, thereby amplifying the power of each signal (in a wavelength band of 1.55 xcexcm or 1.58 xcexcm) propagating through the EDF.
In a typical optical communication system, the gain spectrum of the EDFA is not flat in the communication wavelength band including the individual signal channels, so that a gain equalizer is utilized together with the EDFA. This gain equalizer has a loss spectrum with a form substantially identical to that of the gain spectrum of the EDFA. Thus, the optical communication system is designed such that the total spectrum formed by the gain spectrum of the EDFA and the loss spectrum of the gain equalizer becomes flat in the communication wavelength band.
The EDFA outputs not only the power-amplified signal but also noise light such as amplified spontaneous emission (ASE) and pumping light. If noise light is transmitted, the possibility of reception errors occurring at the time of receiving signals at a receiving end will increase. Therefore, a noise light blocking filter for blocking these noise light components is also provided.
The inventors studied the conventional techniques mentioned above and, as a result, have found the following problem. Namely, since the gain equalizer and noise light blocking filter are provided together with the optical amplifier in the conventional optical communication system, repeaters and the like including them are large in size and expensive.
In order to overcome the above-mentioned problem, it is an object of the present invention to provide an optical filter which functions not only as a gain equalizer for equalizing the gain of an optical amplifier but also as a noise light blocking filter, and has a structure for enabling repeaters and the like to reduce their size and cost together with the optical amplifier; an optical amplifier system including the same; and an optical communication system including the optical amplifier system.
For achieving the above-mentioned object, the optical filter according to the present invention has respective transmission characteristics different from each other in a communication wavelength band, a first blocking wavelength band having a bandwidth of 20 nm or more located on a shorter wavelength side than the communication wavelength band, and a second blocking wavelength band having a bandwidth of 20 nm or more located on a longer wavelength side than the communication wavelength band Namely, the transmission spectrum of the optical filter has a ripple of 1 dB or more with respect to light in the communication wavelength band directed from an input end to an output end, and a transmissivity of xe2x88x9210 dB or less with respect to light in the first blocking wavelength band and light in the second blocking wavelength band which are directed from the input end to the output end.
The optical amplifier system according to the present invention comprises an optical amplifier, and an optical filter having the transmission spectrum mentioned above (the optical filter according to the present invention). The optical amplifier amplifies respective signal powers of a plurality of channels included in the communication wavelength band. The optical filter equalizes the gain of the optical amplifier in the communication wavelength band. The optical communication system according to the present invention is an optical communication system for transmitting respective signals of a plurality of channels included in a communication wavelength band, and includes an optical amplifier system having the structure mentioned above (the optical amplifier system according to the present invention). The optical amplifier system amplifies respective signal powers of a plurality of channels included in the communication wavelength band.
The first and second blocking wavelength bands do not overlap the communication wavelength band, whereas each of the gaps between the wavelength bands (the difference between the maximum wavelength of the first blocking wavelength band and the minimum wavelength of the communication wavelength band, and the difference between the minimum wavelength band of the second blocking wavelength band and the maximum wavelength of the communication wavelength band) is set to 10 nm or less, preferably 5 nm or less, more preferably 2 nm or less. Though it will be sufficient for each of the first and second blocking wavelength bands to have a bandwidth of 20 nm or more in order to block ASE, the bandwidth is preferably 40 nm or more in order to block the light included in L band as well, 60 nm or more in order to block the pumping light (having a wavelength of 1480 nm) as well, and 80 nm or more in order to block multiplexed pumping light (having a wavelength ranging from 1450 nm to 1480 nm) as well.
In the optical communication system employing the optical filter according to the present invention, fluctuations among signal channels are reduced (the gain is equalized) by the optical filter in signals once amplified by the optical amplifier. In particular, the optical filter according to the present invention exhibits a transmissivity of xe2x88x9210 dB or less with respect to light in the first and second blocking wavelength bands not overlapping the communication wavelength band, whereas each of the first and second blocking wavelength bands has a bandwidth of 20 nm or more, whereby at least the ASE generated in the optical amplifier is blocked by the optical filter. Namely, the optical filter according to the present invention functions not only as a gain equalizer but also as a noise light blocking filter. Therefore, as an optical amplifier system including the optical filter, a system having a small size with a low cost can be realized. The optical filter may be arranged either downstream or upstream of the optical amplifier. In the case of an optical communication system in which a plurality of stages of optical amplifiers are arranged, it will be sufficient if the optical filter is arranged between the plurality of stages of optical amplifiers.
Specifically, a first embodiment of the optical filter according to the present invention comprises an optical circulator having first to third ports, and a waveguide type grating device. In the circulator, light from the input end is captured from the first port and is outputted from the second port. Light captured from the second port is outputted from the third port. The optical waveguide type grating device is optically connected to the second port of the optical circulator, and reflects a light component in the communication wavelength band while transmitting therethrough light components in the first and second blocking wavelength bands in the light having arrived from the second port. In thus configured optical filter according to the first embodiment, the light from the input end is outputted from the second port of the optical circulator by way of the first port and reaches the optical waveguide type grating device. In the light having reached the optical waveguide type grating device, most of the light components in the first and second blocking wavelength bands are transmitted through the optical waveguide type grating device On the other hand, most of the light component in the communication wavelength band is reflected by the optical waveguide type grating device, and is outputted from the third port of the optical circulator by way of the second port. Therefore, the transmission spectrum of the optical filter with respect to the light directed from the input end to the output end has a form substantially the same as that of the reflection spectrum of the optical waveguide type grating device
A second embodiment of the optical filter according to the present invention comprises an optical waveguide type grating device arranged on an optical path between input and output ends. In the light having arrived from the input end, the optical waveguide type grating device in the second embodiment transmits therethrough the light component in the communication wavelength band while reflecting the light components in the first and second blocking wavelength bands. Preferably, an optical isolator is provided between the input end and the optical waveguide type grating device. The optical isolator transmits therethrough the forward light directed from the input end to the optical waveguide type grating device, while blocking the backward light directed from the optical waveguide type grating device to the input end. In thus configured optical filter according to the second embodiment, light from the input end is transmitted through the optical isolator, so as to reach the optical waveguide type grating device. In the light having reached the optical waveguide type grating device, most of the light components in the first and second blocking wavelength bands are reflected by the optical waveguide type grating device The light reflected by the optical waveguide type grating device is blocked by the optical isolator, so that it does not return to the upstream side. On the other hand, most of the light component in the communication wavelength band is transmitted through the optical waveguide type grating device, so as to reach the output end. Therefore, the transmission spectrum of the optical filter with respect to the light directed from the input end to the output end has a form substantially the same as that of the transmission spectrum of the optical waveguide type grating device.
A third embodiment of the optical filter according to the present invention comprises a dielectric multilayer film filter which reflects or absorbs light components in the first and second blocking wavelength bands while transmitting therethrough the light component in the communication wavelength band in light having arrived from the input end. Preferably, the dielectric multilayer film filter is arranged in a state inclined with respect to the optical axis (coinciding with the traveling direction of the light having arrived) In thus configured optical filter according to the third embodiment, light from the input end reaches the dielectric multilayer film filter. In the light having reached the dielectric multilayer film filter, most of the light components in the first and second blocking wavelength bands are reflected by the dielectric multilayer film filter Thus reflected light does not return to the upstream side, since the dielectric multilayer film filter is inclined with respect to the optical axis. On the other hand, most of the light component in the communication wavelength band is transmitted through the dielectric multilayer film filter, so as to be outputted from the output end. Therefore, the transmission spectrum of the optical filter with respect to the light directed from the input end to the output end has a form substantially the same as that of the transmission spectrum of the dielectric multilayer film filter.